Amplifying device



May 27, 1941. I M, MbFARLAmE 2,243,533-

AMPLIFYING DEVICE Filed Sept. 16, 1936 2 Sheets-Sheet -l May 27, 1941. M.ID. MCFARLANE I 2,243,533

AMPLIFYING DEVICE Filed Sep t. 16, 1936 2 Sheets-Sheet 2 Patented May 27, 1941 UNIT-ED sires PATENT lorries AMI-PLIFYING DEVICE Maynard DJMcFarIane, Hollywood, Calif.

Application September 16, 1936, Serial No. 101,115

6 Claims.

My invention relates toamplifyingdevices as used in the electronic arts, and may employielectronic devices therein. My invention relates broadly to apparatus adapted for use in electronic amplifiers such as are used in amplifying weak signals of telephone, radio, facsimile, sound picture, and allied industries.

An object of this invention is to provide means whereby vacuum tubes used in amplifiers of the type described maybe operated at a high degree of efiiciency, and whereby the distorting effects of certain interstage coupling devices may .be eliminated.

Another object of my invention is the provision of a circuit whereby a controlled intensification effect is secured, thus increasing efficiency of the amplifier. A further object of the invention is the elimination of certain parts as used in standard amplifying devices, and the consequent cheapening and simplification of the completed apparatus.

Other objects of my invention will be set forth in the following description and drawings, which illustrate and describe preferred embodiments thereof, it being understood -.that the general F statement of the objects of this invention is intended merely to explain the same generally, and not to limit it in any manner.

In carrying this invention into efiect, use may be made of electronic devicesand other component parts of an amplifier of the normal type. These components are however arranged'in' a novel and useful. manner to produce the special results achieved. The invention therefore resides in the arrangement of parts'andin the circuit, and not in the components utilized.

Circuit. arrangementsaccording to this invention provide a method of intensifying the action of a vacuum tube. This intensification may take place in the tube itself, in one or more auxiliary tubes, or in the arrangement of tubes in cascade formation in a circuit which causes each tube to perform its normalfunctions and also to act as an intensifier for the other tubes in the circuit.

The simplest "form of this invention, which is here described with reference to twotubes, although a single multi-element tube may be utilized, resides in making use of thevoltage drop through a resistor or, other suitable element to control the functioning of the circuit in sucha Way as to intensify the normal action ofv the tube.

"For example, suppose that a tricde is wired up in a normal manner -with-the input; and output circuits.

trio-de may be arranged the anode of a 5 second triode, the grid-ofwhich may be carried toa -portion -ofthe circuit which-willrespond to a variation in-anode voltage.

The application of -a signal to the, grid-of the firsttriodechanges the internal resistance-of the tube, andhence the voltage distributionin "the anode supplyc-ircuit which contains at least' one element -having--a=potential difference across it. The effect Qf-achangein the internal resistance of the first triodeis-to-change the voltage drop 'across'the above mentionedcircu-it element. By the arrangement of the circuit, this change of voltage is reflected to thegrid ofthesecondtri- -ode-in such a manner thatthe actionof the second triode changes-the signal output of the system. V

-Thetwo-above;mentioned -triodes may be contained in-one envelope. Alternatively, one of the grids of a-multi-element tube-may be utilized in conjunction with a common anode'to perform the intensifying action here outlined. Due to the large'number of arrangementspossible with multi-element-tubes, no-attempt is made to describe all these'variations. It is-herepointed out that the invention is not limited to the use of sep-- arate tubes for theperformance of its several functions; the structures may be combined in any desiredmannerfor carrying theinvention into effect.

In cases in which electron tubes are connected in .series,the succeeding tubesumay performin ply may be connected. tothe cathode of the first.

tube, and. the. cathodes of the intermediateand lasttubes maybe .fioating and have no cathode connection directly to the battery ,or .;other source of supply.

Avaouumtube may beconsidered for the purpose of, example as a controlled unidirectional; re-

sistor (this is. the normal function of avacuum I tube: with direct current applied, to the anode). In this exampleaof the inyention a, -plura1ity;.0f vacuum tubes are, arran ed; in; series, 'so that: the

'In-parallel --withthe anode" of 4 this r are equal to each other.

equivalent direct current circuit may be considered to be a plurality of resistors across the anode supply. The value of each of these resistors is a function of the grid bias of each individual tube, and in one embodiment of this invention, self-biasing resistors are used whereby the bias of the tube is dependent upon the amount of anode current flowing in that tube.

It can thus be seen that the anode-cathode potential across each tube in the amplifier is a composite function of the internal resistance of that tube and of other tubes in the cascade arrangement. Thus if there are three tubes in cascade, having internal resistances of R1, R2, R3, and a potential E is applied across the amplifier, the working potential of the varioustubes will be expressed by the relationship:

and the anodecurrent of each tube will be identical to the current in each of the others, and equal to any one of the above four expressions. It is thus seen that by control of the internal resistances of each tube, the Working potential of that tube and of theother tubes in the amplifier can be varied. This factor gives an intensifying effect to the amplifier, which under proper control, greatly increases the efficiency of the whole unit. This may be observed by considering the application of a signal to the grid of the first tube of the three stage amplifier above referred to; as the grid potential varies, the internal resistance of the tube is caused to vary, and this in turn affects the other tubes in the circuit for the reasons above given.

For example, suppose that at a given instant the grid of the first tube is more negative than under normal static conditions; less anode current can flow through the tube, and the effective internal resistance of the tube is raised. Therefore, a greater proportion of the total anode voltage E is absorbed by this one tube, and the effective anode potential of th other two tubes are reduced in conformity with the requirement that the anode current of all three tubes shall be the same. Thus independently of any signal arriving at the grids of the succeeding tubes, the current output of those tubes is reduced by an amount which is a function of the negative signal applied to the grid of the first tube.

The circuit arrangement of this invention pro vides for an amplified signal being applied to the grid for each succeeding tube from the anode of the preceeding tube. It can thus be seen that the amplification of a signal is dependent upon two factors; (1) the normal grid controlled amplification of each stage; and (2) the intensifying action caused by the interdependence of the anode potentials.

It can be readily seen that this intensifying action requires careful control to prevent instability of operation. One of the objects of this invention is to provide a circuit which is so arranged as to render this amplifier inherently stable. This is efiected in the design of the amplifier by carefully proportioning the electrical constants of the circuit so that operation is secured within the stable range of the amplifier.

It has above been stated that the circuit is so arranged that the anode currents of all the tubes Should it be desired in any particular arrangement to use a tube of different current characteristics to the other tubes arranged in series with-it, as for example in an amplifier incorporating a power tube as the last tube of the series, this diiference in current consumption of the tubes is accommodated by the use of a bleeder or by-pass resistor. This resistor is so arranged as to be in parallel with the anode-to-cathode path of the tubes drawing the smaller amount of anode current, and in series With the tube drawing the larger amount of anode current.

Under these conditions, in the above mentioned example Where the internal resistances are R1, R2 and-R3, in which the tube with internal re- 7 sistance R3 normally passes a greater amount of current than the others, and a by-pass resistor Riis arranged across the tubes whose resistances arev R1 and R2. The total resistance of this combination is given by the formula:

The current through the tube whose internal resistance is R3 is obtained from the potential drop across that tube or from the total potential drop by one of the following formulae:

=E=E=i 1 2 l+ 2 and the current through the resistor is:

The following relationships also hold:

= E Le R1+R2 r R3 Where it is so desired, this arrangement of tube circuits may be adapted to push-pull circuits. Where a push-pull stage is added at the latter end of a voltage amplifier the above arrangement including the by-pass resistor is usually required due to the greater anode current drawn by the push-pull tubes.

In the above description, no mention has been made of the method of coupling between tubes. I have determined by practical tests that any of the standard types of coupling may be used in an amplifier built according to this invention. Resistance, impedance, and transformer coupling have been used singly and in combination in the construction of amplifiers according to this invention. As in conventional amplifiers, the type of coupling used between the stages is determined by external circumstances such as frequency response required etc., and the type of coupling does not influence the general principles above outlined,

As most conveniently used, this amplifier is arranged for self-bias. The circuit lends itself to such a construction in view of the varying potentials of the cathodes of the tubes. In the resistance and transformer coupled arrangements, a cathode resistance by-passed by a suit-. able condenser may be used as in normal practice; in the impedance coupled arrangement, use

may be made of the potential drop occurring in the inductance due to its direct current resistance to obtain either part or all of the-necessary bias. 7

It will be noted that, in the impedance coupledarrangement, the tubes of the amplifier are'directly coupled one to another. By proper selection of the constants of the circuit, this arrangement lends itself admirably to the use of the amplifier for amplifying small direct cur rents.

In calculating the constants of circuits according to this invention, I have found it convenient to determine the required anode currents of the amplifier and then to determine the total anode potential required in order to give the correct current flow according to one of the above formulae. Where the potential drop due to the direct current resistance of the impedance, resistance, or transformer is any appreciable amount, the total applied potential to the cascade arrangement may be increased by an amount equal to the'sum of the individual coupling device losses in order that each tube may receive its correct voltage as previously determined.

Where filament type tubes are used in carrying this invention into efiect, separate filament supplies may be required for each tube of the cascade arrangement, as these filament are at varying potentials. However, when heater type tubes are used, this separate supply to each heater may not be necessary, as the circuit is from anode to cathode, and this may be made part of the circuit independently of the heater potential. Therefore, preferably, I employ heater tubes in carrying this invention into effect, but it must be noted that filament type tubes may be substituted therefor as and when desired.

In View of the foregoing, it should be noted that filament and heater type tubes may be mixed in an amplifier built according to this invention. For example, one stage of the amplifier may employ a filament type tube or tubes, and the remaining stages may be heater types energized from the same source of supply. This may conveniently be the case where it is desired to employ a filament tube or tubes in the power stage; in this case the voltage amplifier tubes may be heater type; due to the arrangement above outlined, the cathodes of the voltageamplifier tubes will be negative with respect to'the heaters thereof which will tend to reduce any inclination to cathode hum where alternating current is employed on the heater elements.

Certain constructions according to my invention are herewith illustrated by way of example and to explain the same in a general manner. The scope of this invention is not limited to the particular constructions shown in the accompanying drawings, in which:

Figure 1 is a schematic circuit of a single stage amplifier.

Figure 2 is a schematic diagram of an impedance method of coupling.

Figure 3 is a schematic diagram of another form of impedance coupling.

Figure 4 is a schematic diagram of a form of resistance coupling.

Figure 5 is a schematic diagram of a form of transformer coupling.

Figure 6 is a schematic diagram of a cascade amplifier with resistance and impedance coupling.

Figure 7 is a schematic diagram of a cascade amplifier using impedance and transformer cou pling. a I

Figure 8 is a schematic diagram of a cascade amplifier using impedance coupling with tubes of diiferentcurrent characteristics.

Figure 9 is a schematic diagram of a cascade amplifier employing a push-pull stage.

Figure 10 is a schematic diagram of. a cascade amplifier with two heater type tubes and one filament type tube. For purposes of illustration, the cascade amplifiers shown in the drawings have three tubes arranged in cascade. This invention is not limited however to theuse of an arrangement of three tubes; two or any greater number of tubes may be arranged in this manner as desired. In practice I have constructed amplifiers using from two to five tubes in accordance with this invention, and there is no reason for limiting the number of tubes in this type of amplifier other than that applying to all types of vacuum tube amplifiers.

The drawings illustrate the use of this invention in connection with what are commonly termed audio frequency amplifiers. By use of the proper coupling arrangements, the circuits of this invention may be applied to intermediate frequency and radio frequency amplifiers. This change of species is so well known toth-e art that it is unnecessary to describe the change of coupling devices here, it being understood that the scope of this invention includes other types of amplifiers than the specific audio frequency amplifiers illustrated, and that the term amplifiers as used in this specification is not limited to amplifiers in the audio frequency range.

Furthermore, the examples of this invention here illustrated are shown in connection with triodes only. This is again done for purposes of example, as the principles here laid down apply equally well to multi-element tubes. In practice, I have operated pentodes and other multielement tubes in amplifiers of this type.

There are however, so many other types of tubes that it. would not be practical to illustrate all possible combinations, and it is to be understood that the illustration of the circuits employing triodes does not in any way limit the scope of the invention to that particular type of shown at I, and the heater of the tube is energized by the battery F. Anode supply is provided by the battery E through an element G, which may be of any desired type to produce a voltage variation of the required value across the element G due to changes in the circuit.

In parallel with the anode of the tube A is connected the anode of the triode B, which may be provided with a heater supply from the same battery F. The cathode of the triode B may be connected to a point of the resistor H, which is arranged across the element G to ground, and the grid of the tri-ode B may be connected to ground. The negative of the battery E may also be connected to ground; The output transformer of the circuit is connected at the point K between the element G and resistor H.

It can thus be seen that the element G and the resistor H provide a voltage divider across the battery E, whereby the voltage supplied to the anodes of the triodes A and B' is varied in accordance with the current drain of these triodes. The values of the element G and resistance H are calculated in conjunction with the values of the transformer D and the internal resistances of the tri-odes A and B so as to give an optimum and stable operating condition.

Let it be considered that the gridof the triode A goes negative under certain circumstances of input signal. The internal resistance of the tube A is raised, the current drain is lowered, and the current through the element G is thereby decreased. The voltage drop across. the element G is thus decreased, with a corresponding increase of the voltage drop across the resistor H. The grid of the triode B is thus rendered more negative and its internal resistance is increased; and by this action the effect of the tube A on the output circuit is intensified by the amount of change in the tube B.

. While a self-biasing resistor is shown in the circuit, and in the other circuits illustrated here.- in, these may be replaced by biasing batteries without changing the performance of the system in accordance with my invention. Similarly the batteries shown for filament and anode supply may be replaced by other forms adapted to supply the requisite currents and potentials without in any way departing from the principles here laid down. Similarly, the two triodes shown here may be included in one envelope, or may be replaced by a multi-element tube as desired.

The element G may be a resistor, or it may be a network of any desired characteristics for the control of the amplifier. By having discriminatory frequency characteristics incorporated in the element G, control of the frequency response of the amplifier may be secured through the controlled action of the tube B by this means. The network need not of necessity be passive, and thus a large variety of controlled results can be obtained, as for example AVC and similar desired operationalfeatures.

The remaining figures illustrate cascade arrangements of tubes in which the successive tubes perform the functions of the element G in addition to those of dynamic amplification in the ordinary manner. trate different methods of coupling two tubes when used in an amplifier built according to this invention. These tubes are, for purposes of illustration, shown as triodes, although other types of tubes may be used. V1 represents the first tube of the combination and V2 the second tube;

In Figure 2 the anode I9 of the tube V1 may be coupled to the grid I2 of the tube V2 by the impedance II. This impedance II may be a choke having any desired characteristics. The anode III of the tube V1 is joined directly to the grid I2 of the tube V2, and also to the top of the impedance II. The lower end of the impedance H is connected'to the cathode I 4 of the tube V2 and to one side of the condenser I5. The other side of the condenser I5 is connected to ground.

In this modification of the invention, the necessary bias for the grid I2 of the tube V2 with respect to its cathode I4, is derived by the potential drop in the impedance I I due to its direct current resistance. The condenser l5 serves to by-pass the signal current to ground and to stabilize the operation of the circuit. This condenser serves to prevent excessive and uncontrolledbuilding-up of signals due to reflex intensification. The value of capacity used is dependent upon other circuit parameters, such as.

tube impedance, load, etc., and also upon the de- Figures 2, 3, 4, and 5 illussired frequency response characteristic of the amplifier: in practiceit has been found convenient to use a value which gives a time constant for the circuit corresponding to a frequency below the useable range of the amplifier. As previously explained, the operating potential for the combination is applied to the anode I6 of the tube V and to the cathode I! of the tube V1 either directly or through other apparatus as will be shown later. v

Figure 3 shows a variation of the impedance coupling illustrated in Figure 2. In this case, the direct current resistance of the impedance I8 is insufficient to furnish the correct bias for the grid I2 of the tube V2. Therefore between the lower end of the impedance I8 and the oathode M of the tube V2 is inserted a resistor I9 shunted by a by-pass condenser 20. The sum of the resistances of the resistor I9 and impedance I8 are such as to provide a correct bias for the grid l2 of the tube V2 in accordance with the well known principles appertaining to the use of self-biasing resistors.

Figure 4 illustrates one method of utilizing resistance coupling according to this invention. The anode III of the tube V1 is connected to the top end of the plate resistor 2I and to one side of the coupling condenser 24. The other side of this coupling condenser 24 is connected to the top end of the grid resistor 22 and the grid I2 of the tube V2. The lower ends of the resistors 2| and 22' are connected to the cathode M of the tube V2 through a self-bias resistor I9, which latter is equipped with a by-pass condenser 20,'and to'one side of the by-pass condenser I5, the other side of which is connected to ground.

The grid bias for the grid I2 of the tube V2 is controlled by the value of the resistor I9 as in conventional self-biasing circuits. The alternating current signal coming through the tube V1 causes a potential variation on the coupling condenser 24 due to the variation in current flowing through the resistor 2|. This variation in potential is'applied to the grid I2 of the tube V2 in the usual manner in resistance coupled circuits. The grid I2 of the tube V2 is prevented from accumulating a charging potential by means of theresistor 22, and is kept in its proper operating bias by the resistor I9 as previously described. The condenser I5 by-passes the signal voltage to ground and stabilizes the action as in the previous cases.

In Figure Stheresistance and condenser coupling network of the Figure 4 is replaced by a transformer 21 with its primary 25 in the anode circuit of the tube V1 and its secondary 26 in the, grid circuit of the tube V2. As described in connection with Figure 4, the bias for the grid I2 of the tube V2 is obtained by the correct proportioning of the resistor I9. The lower ends of the primary 25 and secondary 26 of the transformer 21 are joined together and are connected to the resistor I9 with its by-pass condenser 20 and to one side of the by-pass condenser ,I5, the other side of which is connected to ground. The alternating current signal from the anode ID of the tube V1 passes through the primary 25 of the transformer 21, and generates a signal in the secondary 26 of the transformer 21, which signal is applied to the grid I2 of the tube V2.

Figure 6 illustrates a cascade amplifier employing three tubes, V3, V4, and V5. For purposes of illustration, the coupling between the tubes V3 and V4 is shown as resistance coupling in .accordance'with Figure 4, and the coupling between V4" and V5 is impedance coupling in accordance with that shown in Figure 2.

In this modification, the signal is applied to the primary 3! of the input transformer 34, the secondary 32 of which is connected to ground and to the lower end of the secondary 32 of the transformer 35 by means of the bias resistor 38 and by-pass condenser 39. The anode 35 of the tube V3 is connected to one end of the plate resistor and to one side of the coupling condenser 4|. The other side of the coupling'condenser 46 is connected to one side of the grid resistor 42 and to the grid 44 of the tube V4. The other ends of the resistors 40 and 42 are connected to one side of the by-pass condenser 43 and to one side of the grid bias resistor 48 and its by-pa-ss condenser 49. The other side of the grid bias resistor 48 and its by-pass condenser 49 are connected to the cathode 46 of the tube V4, which is heated by the heater 4?. The other side ,of the by-pass condenser 43 is connected to ground.

The anode of the tube V4 is connected to the grid 54 of the tube V5, and also to one side of the impedance 50. This impedance is so selected that its resistance provides adequate bias for the grid '54 of the tube V5. The other end of the impedance 50 is connected to the cathode 56 of the tube V5, which is heated by the heater 51. The cathode is also connected to one side of the by-pass condenser 53, the other side of which is connected to ground.

The anode of the tube V5 is connected to one side of the primary 60 of the transformer Bl. The secondary 62 of the transformer 6| comprises the output of the amplifier. The other side of the primary-'60 of the transformer BI is connected to the positive terminal of the anode supply battery 64, the negative side of which is connected to ground. It is thus seen that the only anode supply to the amplifier is from the battery 64, the positive of which is applied to the anode 55 of the tube V5 through the primary 60 of the transformer 61, and the negative of which is applied to the cathode 36 of the tube V3 through the ground .connection of both cathode and battery.

The signal path through the amplifier is from the primary 3| of the transformer 30, to the secondary32 and thence to the grid 34 of the tube V3 and to ground. From the anode 35 of the tube V3 through the resistor 40 and condenser 43 to ground, also through the coupling condenser 4| to the grid 44 of the tube V4 and i through the cathode 45 of the tube V4 through the condenser 43 to ground. From the anode 45 of the tube V4 to the grid 54 of the tube V5, and through the cathode 56 and condenser 53 to ground. From the anode 55 of the tube V5 to the primary 50 of the transformer 6| and through the battery 64 to ground, and from the secondary 62 of the transformer 6! to the output.

In this embodiment of the invention, the three tubes V3, V4, and V5 are so selected that current carrying capacities under normal static conditions are the same, and the potential of the battery 64 is so selected as to secure this flow of current through the three tubes in series and through the resistors 38, 40, 48, and the resistances of the impedance 50 and the primary 60 of the transformer 61. These resistances are added to the internal resistances of the, tubes V3, V4, and V5 in computing the total resistance of the circuit through which it is desired to pass the predetermined amount of current by means of the battery 64.

For example, let it be supposed that each of the tubes passes a current of 5 milliamperes under given static conditions, and that a bias resistor of 2500 ohms is required. to secure the correct grid bias for each tube under these conditions. Resistance of the impedance 50 should, under these circumstances be 2500 ohms. Let it be' assumed that the internal resistance of each tube under these conditions is 10,000 ohms, and that the value selected for the resistor 40 by conditions of circuit design is 50,000 ohms, and that the resistance of the primary 60 of the transformer 6] is 500 ohms. Then the total resistance in the circuit, calculated as above described, is 30,000 ohms (10,000 for each tube), plus 5,000 ohms for the two resistors 38 and 48, plus 2,500 ohms for the impedance 50'plus 500 ohms for the primary 60 of the transformer 6 l, plus 50,000 ohms for the resistor 40, giving a total of 88,000 ohms. This will require a voltage of 440 volts in the battery 64.

In calculating the above voltage conditions, arbitrary values have been used as the calculation is intended merely as an example of the method employed in determining the voltage to be applied to the complete amplifier. The actual values to be employed are parameters of'the tubes selected and are also functions of the determined amplifier characteristics as is well known in the art.

The heaters 31, 41, and 51 of the tubes V3, V4, and V5 are energized from any convenient source, either alternating or direct current and they may be arranged with the heating elements either in series or parallel as may be dictated by the conditions of supply. As in each tube the cathode is insulated from its associated heater (36 is insulated from 31, 43 from 41, and 56 from 57) the potential at which the heaters are operated with respect to the rest of the system has no' vital bearing on the performance of the amplifier provided that the potential difference between cathode and heater in no case 'is allowed to exceed the operational maximum for that type of tube.

In Figure 7 is shown another modification of a cascade amplifier employing the principles of this invention. The input from the primary 6 6 of the input transformer and secondary 68 to the tube Vs are identical with the same circuits for tube V3 in Figure 6. The anode 65 of the tube V6 is however coupled to the grid 14 of the tube V: in the manner shown in the Figure 3. The anode 65 of the tube V6 and the grid 14 of the tube V7 are connected to one endof the impedance 1!]. The otherend of the impedance T0 is connected to one side of the condenser 13, the other side of which is connected to ground, and to one side of the resistor 18 and one side of its associated by-pass condenser 19. The other side of the resistor 18 and by-pass condenser 13 are connected to the cathode 15 of the tube V7. v The tube V7 is coupled to the tube V8 by a transformer coupling as illustrated in Figure 5. The anode 75 of the tube V7 is connected to one side of the primary 8'! of the transformer 80. The other side of the primary BI and one side of the secondary 82 of this transformer are connected to one side of the condenser 83, the other side of which is grounded, and to one side of the bias resistor 88 and its associated byv pass condenser 89. The other sides of the resistor 88 and condenser 89 are connected to the cathode 86 of the tube Vs. The top end of the secondary 82 of the transformer 80 is. connected to the grid 84 of the tube Vs.

The anode 85 of the tube Va is connected to the output transformer 9| through its primary 90 in an exactly similar manner to that shown for the transformer BI in Figure 6. The battery 94 is connected in exactly the same manner and performs the .same function for the amplifier shown in Figure 7 as the battery 64 for the modification shown in Figure 6, and the potential of the battery 94 is calculated in a similar manner to that of the battery 64.

The heaters 61, 11, and 81 of the tubes V6, V1, and V3 may be energized in any suitable manner as described for the heater of the amplifier shown in Figure 6.

It will be noted that in the two amplifiers illustrated by way of example in Figures 6 and '7 the four methods of interstage coupling illustrated in Figures 2 through 5 have been utilized. These four methods of inter-stage coupling may be combined in any suitable way, and the amplifiers shown in Figures 6 and '7 do not limit the combinations of coupling methods which may be used.

In Figures 6 and 7, amplifiers have been illustrated in which the tubes used draw identical currents, that is, the tubes may be selected so that there is a common current value for all the tubes of any one amplifier around which the tube may be used to perform the functions desired. This does not necessarily mean that the tubes must be identical; and in certain designs of amplifiers it may be desirable to have tubes operating in other points than the mid-points of their characteristic curves. These factors may be taken into consideration in selecting the tubes to be used in an amplifier of the class illustrated in the Figures 6 and '7.

In many types of amplifier it is desirable to have an output tube of totally different characteristics to the tubes used in the earlier stages, and usually such output tubes draw a greater amount of anode current and have a lower internal resistance than the tubes normally used in the earlier stages. According to my invention, I can compensate for a difference of anode current drawn by such a tube by means of a bleeder or by-pass resistor. This resistor may be arranged in parallel with the anode-cathode circuits of the remaining tube or tubes in the amplifier.

By way of example, such an amplifier is shown in Figure 8 where three tubes V11, V111, and V11 are shown coupled in the manner illustrated in Fig ure 2. The input to the primary 96 of the input transformer is passed through the tubes V9 and V10 in the manner previously described. The anode I05 of the tube V10 is connected to the grid II4 of the tube V11 and also to one end of the impedance I00. The other end of the impedance I is connected to the condenser II3, the other side of which is grounded, and also to the cathode IIB of the tube V11, and to one side of the bleeder resistor H8, the other side of which is connected to ground. The anode II of the tube V11 is connected to the primary I20 of the output transformer I2I as previously shown with transformer BI in Figure 6. The battery I24 applies anode potential to the system. As the tube V11 in this example draws a greater amount of current than the tubes V9 and V111, this excess current is by-passed through the resistor H8. In other words, the sum of the currents through the branch of the circuit containing tubes V9 and V111 and the branch containing resistor H8 must equal the current through the tube V11. The branch containing the tubes V9 and V10 carries a current determined by the tubes, and the potential necessary for this is calculated as in the previous examples. With this potential and current value known, the value of the resistor II8 can then be calculated, and from that can be determined the potential required in the battery I24.

It is frequently desired to arrange the output stage in a push-pull arrangement. This is possible according to my invention, and one form of such an arrangement is illustrated in Figure 9. In this figure four tubes are shown, tubes V12 and V13 being voltage amplifier tubes coupled in the manner shown in Figure 4. Tubes V14 and V15 are the push-pull tubes, and they are coupled to the tube V13 by means of the push-pull transformer I40. It should be noted that the tubes V14 and V1:1 need not be separate tubes, but may consist of separate halves of a double tube, such as the type known to the trade as 6A6.

The anode I of the tube V13 is connected to one end of the primary I4I of the transformer I40. The other end of the primary I M is connected to one side of the condenser I43, the other side of which is grounded, and is also connected to the center tap of the secondary winding I42 of the transformer I40. It is also connected to one side of the bleeder resistance I58 and to one side of the bias resistor I48 and its associated by-pass condenser I49. The other sides of the bias resistor I48 and of the by-pass condenser I49 are connected to the cathodes I46 and I56 of the tubes V14 and V15. The other side of the resistor I58 is connected to ground, and this resistor performs the same function and is determined in the same manner as-described in connection with the resistor H8 in Figure 8. The resistor I48 performs the same function as the resistor III in Figure 5, and iscalculated in the same manner, due allowance being made for the fact that'it is in the circuit of both tubes V14 and V15.

The secondary I42 of the transformer Ihas its center tap connected as described for the lower end of the coupling transformer in the previously illustrated cases, and the endsare connected respectively to the grids I44 and I54 of the tubes V14 and V15. The anodes I45 and I55 of the tubes V11 and V15 are connected in a conventional manner to the primary I of the output transformer I5I. The center tap of the primary I50 is connected in the conventional manner to the battery supply I53, the two outer ends of the primary going to the anodes of the tubes.

The potential of the battery I53 is calculated in a similar manner to that previously outlined in connection with the other circuits. The heaters I21, I31, I41, and I51 are connected in any desired manner to a suitable source of cur-' rent supply, or if desired they may be connected to difierent sources of supply. In none of the above circuits is it in any way essential that one source of heater current shall be used for all tubes, although this is the usual practice on grounds of economy, It has previously been mentioned that under certain circumstances it may be desired to employ a filament type tube in one stage of an 1 amplifier built according to this invention. For

purposes of illustration, a circuit employing two heater type tubes and one filament tube is illustrated in Figure 10. The arrangement of the tubes, and the coupling between stages, is identical with that shown in Figure 6. It is therefore unnecessary to repeat the particulars given with reference to Figure 6 in connection with Figure 10.

The heater I61 of the tube V16, the heater I11 of the tube V11, and the filament I81 of the tube V18 are shown wired parallel to the battery I64. This battery I64 supplies the necessary current for bringing the heaters and filament up totheir operating temperatures.

The lower end of the impedance H is connected to a condenser I13, of which the other side is connected to ground, and also to one side of the filament I81 of the tube V18. The tube V18 has no indirectly heated-cathode, and therefore this connection is necessary to replace the connection to the cathode 56 in the tube V of Figure 6.

While the circuits shown herewith by way of illustration employ my novel method throughout, it is to be understood that my invention is not limited in any way to the particular circuits shown here, and that many variations thereof may be made without departing from the spirit of the invention. For example, in an amplifier, stages utilizing the principles of my invention may be used in combination with stages of a conventional type. For example, should it be desired to limit the amount of anode potential applied to an amplifier, and yet to make use of the advantages of this invention, a four tube circuit may be employed in which the first two and last two stages are arranged in accordance with this invention and the coupling between the second and third tube is arranged in a conventional manner. This is mentioned by way of example only, as it is obvious to one skilled in the art that there is a very large number of such combinations possible.

In addition, it is not necessary that the final stage of an amplifier shall be as shown in the figures which are merely given for purposes of illustration. For example, in Figure 6, the secondary 62 of the output transformer 6| may be replaced by a conventional grid circuit secondary, and a power stage, either single or push-pull may be added in the end of the amplifier receiving its heater and filament supply from the same source as the rest of the amplifier and receiving its anode current from the same battery 54.

In practice I have used many such combinations, adding conventional stages ahead of, in between, and after stages built according to this invention. The particular arrangement best suited for any purpose is determined solely by the conditions to be met in the design of an amplifier to accomplish certain definite purposes, therefore no particular combination of special and conventional stages is intended to be herein shown, as the arrangement of the stages will vary in accordance with the individual requirements of design in each case.

Where in this specification and claims I have used the words tube or vacuum tube, it is not intended that I shall be limited to any particular type of tube, vacuum or gaseous. Circuits according to this invention may be employed in connection with gaseous devices, and the phraseology here used is merely that of the conventional language of the art, and the terms refer to electronic devices in general. Furthermore, while the circuits shown have related to class A amplificaameters will allow of their use in class B and C circuits. Furthermore, the invention is not limited'to the audio frequency circuits used by way of illustration, and may be used in connection with any amplification circuits of whatever frequency may be desired by the use of suitable circuit parameters.

While for ease of explanation, all circuits shown in the drawings comprise a plurality of tubes, the principles of the invention can be carried out in a single tube by suitable circuit design. For instance, in Figure 1 the grid of the tube B may be incorporated in tube A, either as a separate element or by a suitable arrangement the existing grid of tube A may be caused to perform the composite function. The scope of the invention is not therefore limited in this respect.

In Figure 1 is shown a means for carrying this invention into effect in a single stage of amplification. In Figures 2 to 10 the invention is illustrated in connection with multi-stage amplification. In all the illustrations given, the operation of a tube is in part controlled by the operating characteristics of the other tubes in the circuit; and thereby the performance of one tube is rendered dependent upon the other tubes in the circuit; the whole arrangement being comprised of interdependent parts to perform the functions as above set forth.

For example, in the arrangement shown in Figure 1 the total amplification of the signal is dependent upon the control exerted by tube B on tube A. The change in internal impedance of tube B causes a change in the characteristics of the circuit which afiects the performance of tube A as an amplifier. In the circuit shown in Figure 6, each tube by a change in its internal impedance afiects the amplification of the other tubes in the circuit, in addition to the amplification caused by the tube itself. In Figure 1, the tube B does not itself amplify the signal, but it controls the amplification performed by tube A; in the remaining figures each tube amplifies the signal as well as controlling the amplification of the other tubes in the circuit.

Having now described my invention, I claim:

1. In a device of the class described, a plurality of electron devices having their anodes connected together, means for applying a signal voltage to the grid of at least one of said electron devices, means for causing a signal-controlled potential variation in the anode supply to the anodes, and means for causing a corresponding potential variation to be applied in an inverse sense to another of said electron devices.

2. In a device of the class described, a plurality of electron devices having their anodes connected to a common point, means for applying a signal voltage to the grid of at least one of said electron devices, means for causing a signal-controlled potential variation in the anode supply to said common point, and a resistive element between said common point of anode connection and the negative of the anode supply, the cathode of one of said electronic devices being connected to said resistor at a point more positive than the grid of the same electronic device.

3. Apparatus for interstage coupling in a multi stage amplifier comprising: a first tube and an anode, an anode load connected between said anode and the cathode of a second tube, and means to apply a signal voltage to the grid of the second tube for signal amplification; said anode load being the primary of a transformer and said coupling means being the secondary of the same transformer; the primary and secondary windings of said transformer being joined together in series-aiding relationship, by connecting to each other the ends of the windings opposite to the ends connected to said anode and said grid; said cathode connection being made connections, the method of coupling the anode of one stage to the grid of a succeeding stagewhereby the grid of a succeeding stage is suitably biased by the potential drop in the coupling device due to the anode current flowing therein.

5. In a multi-stage electronic amplifier having the tubes thereof connected in an anode-to-cathode series arrangement, the anode operating potentials of the tubes being applied only through the tubes themselves in series, means for coupling the anode of one stage to the cathode of a succeeding stage by an inductive element and directly to the grid of the same succeeding stage, whereby the efiective impedance of the said coupling element provides an anode load for the first stage, and the effective direct current resistance of the said coupling element provides a grid bias voltage for said grid.

6. Apparatus for interstage coupling in an electronic amplifier having a'first tube and a second tube comprising: coupling means connecting the anode of the said first tube to the cathode of said second tube, means to apply potential to the two tubes in series only through the said coupling device, means to apply a signal to the grid of the first tube, and to apply a corresponding amplified signal to the grid of the second tube by connection of said second grid to the anode of the said first tube.

'MAYNARD D. McFARLANE. 

